During operation of an image-forming device such as a laser printer or copier, a fuser permanently affixes toner particles to a media sheet in the final stage of a non-impact image-forming process. When fusing toner onto a substrate, the toner is heated to a point where the toner coalesces and appears tacky. The heat allows the toner to flow, thereby enabling it to coat the fibers or pores of the substrate. With the addition of pressure, an improved contact between the substrate and toner can be obtained. The heat and pressure required for fusing are achieved by a heated member and a pressure roller that under an applied force form a nip. Heat from the fuser melts the toner particles, while the applied pressure allows for absorption thereof into the media. Subsequent cooling of the toner, while it is in intimate contact with the substrate, allows it to adhere to the sheet.
The heater of the fuser may be mounted within a movable belt. During operation, stick-slip friction between the heating element and the belt introduces vibration and noise that prematurely wear the components and can affect image quality. While lubrication between the heater and belt can reduce the vibration and noise, it may be difficult to retain such lubrication in the desired location over time, under the heat and pressure of the components during ongoing operation of the device.
This problem is exacerbated over the life of the printer, as over time, the fuser lubricant migrates away from any high-pressure areas present on the heater glass.
One factor that has a significant effect on belt vibration noise is any uneven areas of the heater external surface which is in contact with the belt. Uneven areas of the heater can scrape the grease off the inside of the fuser belt and reduce the film thickness in the areas with high pressure contact. In addition, if the amount of waviness and/or unevenness in the outer surface of the heater is greater than the grease thickness, areas of contact between the outer heater surface and the fuser belt may undesirably operate with boundary layer lubrication and stick-slip may occur, thereby causing vibration and noise.
The conventional screening method used to make ceramic heaters leaves raised portions on the surface in the areas of the resistive traces.
Referring now to FIGS. 3A-3B, there is shown an enlarged, partial, cross-sectional view of a known fuser heater 40. Heating elements 56 are disposed on a base 54 and a protective, covering layer 62 is disposed over heating elements 56 and the corresponding surface of base 54. As can be seen in FIGS. 3A and 3B, the outer surface of covering layer 62 does not provide a substantially even surface due to protruding nature of the heating elements 56 relative to the surface of base 54 as well as to unevenness in the heating elements 56 themselves.
FIG. 3B shows the general shape of the heating elements 56 in the known fuser heater 51 taken at a greater magnification than FIG. 3A. As shown in FIG. 3B, the actual edges of heating elements 56 are slightly raised, creating areas of high contact pressure in covering layer 62, thereby contributing to the unevenness in the outer surface thereof. The outer surface of heating elements 56 have been seen to be substantially meniscus shaped. The amount of unevenness in the outer surface of known heater 51, which can be seen as dimension H in FIG. 3B, has been seen to be greater than 12 μm.
Published references pertaining to fusers, ceramic heaters and related technology include U.S. Pat. Nos. 6,157,806, 6,818,290, 6,865,351, 6,870,140, 6,879,803, and 7,193,180, which are assigned to the assignee of the present application and hereby incorporated by reference into the application in their entirety.
Although the known fuser assemblies have some utility for their intended purposes, a need still exists in the art for an improved heater for a fuser assembly usable in electrophotography, as well as to methods of making such an improved heater.